The present invention relates to a method for detecting misfire based on fluctuation in crankshaft rotation.
If a misfire condition, in which combustion in a cylinder is not performed properly, occurs in the operation of an internal combustion engine due to the failure of fuel injector or the like, the exhaust gas characteristic or the like of the engine is deteriorated. To obviate this, the information corresponding to the number of revolutions or rotation speed is calculated on the basis of the period of each predetermined crank angle corresponding to each cylinder of an engine, and the misfire condition of engine is detected on the basis of the change amount or change rate of this information, as disclosed in Japanese Unexamined Patent Publication No. H2-49955, etc. According to this Patent Publication, if misfire is detected on the basis of the deviation of the rotational angular velocity of the internal combustion engine from the rotational angular velocity before one firing as the reference angular velocity, that is, the rotational fluctuation, which is calculated for each firing interval in synchronization with the combustion stroke in the engine, misfire cannot be detected exactly when random misfire occurs or when misfire occurs approximately once for several revolutions. In Japanese Unexamined Patent Publication No. H2 -49955, therefore, the reference angular velocity is renewed as necessary.
As described above, the period of the predetermined crank angle is detected by using a crank angle sensor. The crank angle sensor typically includes a rotary member (rotor) which has a plurality of vanes or protrusions disposed at equal angular intervals and which is mounted to the crankshaft for rotation in unison therewith, and a detecting portion which is disposed to face the rotary member for detecting the passage of the vane. These vanes of the rotary member protrude in the radial direction at the periphery of the rotary member. The number of vanes corresponds to the number of engine cylinders; for example, the crank angle sensor for a 6-cylinder engine has three vanes. With the crank angle sensor of such a construction, when the end of one of adjacent vanes passes through the detecting portion as the engine rotates, the entering of crankshaft into the crankshaft rotational angle region corresponding to that vane is detected, and when the end of the other vane passes through the detecting portion, the leaving of crankshaft from that region is detected. Thus, the time interval from the time of entering the angle region to the time of leaving the region, that is, the period is detected. Further, the crankshaft rotational speed at the crankshaft rotational angle region is calculated on the basis of the detected period. Then, a determination as to whether misfire occurs is made based on the magnitude of the change in rotational speed.
In such a misfire detection method, the accuracy in detecting the period, that is, the accuracy in detecting misfire depends on the vane angle interval at the crank angle sensor, that is, the length in the vane circumferential direction. Nevertheless, an error in the construction of crank angle sensor, particularly an error in manufacturing and installing the vane, is inevitable, so that variations are produced in the vane angle intervals, thereby the accuracy in detecting misfire being decreased. When the vane angle interval is larger than the design value, the crankshaft rotation at this angle region requires much time. Therefore, it is sometimes judged by mistake that the crankshaft rotational speed has decreased at that angle region, so that the occurrence of mistakenly detected misfire.
More specifically, in the case that the angular acceleration of the crankshaft of a 2-cylinder engine is calculated based on a time interval detected by use of a crank angle sensor having no manufacturing and installing errors, both calculated angular acceleration values corresponding to two cylinders are substantially equal to zero, as shown in FIG. 1, provided that no misfire occurs in the engine and the crankshaft rotates at a constant speed. On the other hand, as shown in FIG. 2, a fluctuation appears in the angular acceleration calculated on the basis of a time interval detected by means of an ordinary crank angle sensor having errors in manufacturing and installing vanes, even if the engine is in a constant speed operation. That is, the calculated angular acceleration associated with one of the cylinders which corresponds to a crankshaft rotational angle region where a vane angle interval is too small has a positive value, whereas the angular acceleration associated with the other cylinder corresponding to another crankshaft rotational angle region where the vane angle interval is too large has a negative value. If the vane angle interval is considerably large, the calculated angular acceleration becomes less than a misfire determination level, as shown in FIG. 2, resulting in an erroneous determination of occurrence of misfire even when actual misfire does not occur.
In the case of calculating the crank angle acceleration based on the time interval detected by means of a crank angle sensor free from vane errors, the calculated angle acceleration, corresponding to a cylinder where misfire occurs, becomes less than a misfire determination level, as shown in FIG. 3, so that occurrence of misfire can be detected accurately. On the other hand, in the case of using a crank angle sensor which has vane errors, occurrence of misfire cannot be detected when the misfire occurs in a crankshaft rotational angle region where the vane angle interval is too small, since the calculated angular acceleration corresponding to the cylinder where the misfire occurs becomes larger than the misfire determination level, as shown in FIG. 4.
The above-mentioned mistaken detection cannot be eliminated by, e.g., the technology of renewing the reference angular velocity which is disclosed in . Japanese Unexamined Patent Publication No. H2-49955.